From ab181ac329bb19bdca5e266bc9fcaed36d5c51fb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Stefan=20Br=C3=BCns?= <stefan.bruens@rwth-aachen.de>
Date: Sun, 26 Dec 2021 00:08:06 +0100
Subject: [PATCH] highgui: Fix unresolved OpenGL functions for Qt backend

The Qt backend directly calls some OpenGL functions (glClear, glHint,
glViewport), but since OCV 4.5.5 the GL libraries are no longer part
of the global extra dependencies. When linking with "-Wl,--no-undefined"
this causes linker errors:

`opencv-4.5.5/modules/highgui/src/window_QT.cpp:3307: undefined reference to `glClear'`

Fixes: #21346
Related issues: #21299
---
 modules/highgui/CMakeLists.txt | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/modules/highgui/CMakeLists.txt b/modules/highgui/CMakeLists.txt
index 0d63d42fea..a7fdfc8b67 100644
--- a/modules/highgui/CMakeLists.txt
+++ b/modules/highgui/CMakeLists.txt
@@ -275,6 +275,10 @@ if(OPENCV_HIGHGUI_BUILTIN_BACKEND STREQUAL "WIN32UI" AND HAVE_OPENGL AND OPENGL_
   ocv_target_link_libraries(${the_module} PRIVATE "${OPENGL_LIBRARIES}")
 endif()
 
+if(OPENCV_HIGHGUI_BUILTIN_BACKEND MATCHES "^QT" AND HAVE_OPENGL AND OPENGL_LIBRARIES)
+  ocv_target_link_libraries(${the_module} PRIVATE "${OPENGL_LIBRARIES}")
+endif()
+
 if(MSVC AND NOT BUILD_SHARED_LIBS AND BUILD_WITH_STATIC_CRT)
   set_target_properties(${the_module} PROPERTIES LINK_FLAGS "/NODEFAULTLIB:atlthunk.lib /NODEFAULTLIB:atlsd.lib /NODEFAULTLIB:libcmt.lib /DEBUG")
 endif()